1. Field of the Invention
The present invention relates to a sheet conveying device which is capable of shifting sheets in a direction intersecting a sheet conveying direction, and an image forming apparatus including the sheet conveying device.
2. Description of the Related Art
Some recent image forming apparatuses have a number of optional units, such as a post-processing unit provided with a stapling function, a punching function, a saddle-stitching function, and like functions, and a large-capacity stacker, connected thereto to thereby construct an image forming system.
In a thus formed image forming system, mounting errors in joint sections between optional units and conveying paths increased in length tend to cause larger positional deviation of sheets (hereinafter referred to as “lateral registration deviation”) in a direction (hereinafter referred to as “the lateral direction”) intersecting a conveying direction than in a conventional image forming apparatus having no optional units connected thereto. The lateral registration deviation is increased particularly in an optional unit disposed at a location downstream, as viewed in the conveying direction, of the image forming apparatus.
Lateral registration deviation causes degradation of positional accuracy in stapling or punching, and hence lateral registration deviation is corrected in the post-processing unit. In order to correct the lateral registration deviation at a high speed, there has been proposed a technique in which the amount of lateral deviation in a conveying position (the amount of lateral registration deviation) is measured in advance, and the lateral registration deviation is corrected by shifting the sheet in the lateral direction while conveying the same.
In this technique, a sheet is shifted in the lateral direction in a state where a sheet shift distance required to correct lateral registration deviation has been grasped. Therefore, it is possible to shift the sheet at an optimal speed according to the sheet shift distance to thereby achieve high-speed correction of lateral registration deviation.
As a typical technique for measuring the amount of lateral deviation in the conveying position, there has been proposed a technique in which an optical transmissive sensor is shifted in the lateral direction, and the amount of lateral deviation in the conveying position is measured based on a distance from a point where the sensor starts to be shifted to a point where the sensor detects a lateral edge of a sheet (see Japanese Patent Laid-Open Publication No. 2005-156578). Further, there is also disclosed a technique for measuring the amount of lateral deviation in the conveying position using a line sensor.
The latter technique makes it possible to achieve high-speed measurement. However, the line sensor and a circuit for controlling the line sensor are very expensive, and hence the former technique is used in a larger number of products.
In the technique disclosed in Japanese Patent Laid-Open Publication No. 2005-156578, a standby position of the sensor is set such that a detecting point of the sensor corresponds to a lateral edge of a sheet conveyed without lateral registration deviation. Then, if the sensor is on at the start of measurement, it is shifted in a direction in which it is to turn off. On the other hand, if the sensor is off, it is shifted in a direction in which it is to turn on.
For example, if it is required to shift the sensor the sensor over a distance of 15 mm in the lateral direction, before the sensor is changed from a state in which a sheet is detected into a state in which the sheet is no longer detected, it is determined that the amount of lateral registration deviation is equal to 15 mm. This means that as the amount of lateral registration deviation is larger, it takes longer for measurement.
Meanwhile, it is demanded that an image forming apparatus be highly productive. With an increase in productivity, spacing (sheet interval) between the trailing edge of a preceding sheet and the leading edge of the following sheet is inevitably reduced, which makes it impossible to secure spacing required for performing post-processing, such as stapling or punching, on sheets. To solve this problem, in order to increase the spacing between a preceding sheet and a sheet following the sheet, it is required to increase the sheet conveying speed.
However, when the sheet conveying speed is increased, it is impossible to detect the amount of lateral sheet registration deviation while a sheet is passing the sensor.
Further, the downsizing of an image forming apparatus is demanded so as to reduce the installation area of the image forming apparatus. However, in a case where detection of the amount of lateral sheet registration deviation and the shifting of a sheet in the lateral direction are performed separately, as in the prior art, it is required to increase spacing in the sheet conveying direction between the sensor and a device for shifting a sheet in the lateral direction, which makes it difficult to reduce the size of the image forming apparatus.